1. Field of the Invention
This invention relates to feed assemblies for reflector antennas. More particularly, the invention provides improvements in reflector antenna feed assembly electrical performance and cost efficiency via a unitary solid dielectric self supporting feed assembly.
2. Description of Related Art
Many broadcast and or communications systems require antennas with a highly directional signal reception and or transmission characteristic. Reflector antennas focus a signal received by a dish shaped reflector upon the feed horn of a centrally mounted receiver. Because the dish shaped reflector only focuses a signal received from a single direction upon the receiver or a sub reflector that further directs the signal to the receiver, reflector antennas are highly directional. When the reflector antenna is used to transmit a signal, the signals travel in reverse, also with high directivity.
Reflector antennas with a sub reflector supported and fed by a waveguide are relatively cost efficient and allow, for example, location of the transmitter and or receiver in an easily accessible location on the back of the reflector. This configuration eliminates the need for a support structure that spans the face of the reflector, partially blocking the reflector, and signal losses associated with passing the signal through an extended waveguide or cable routed along the support structure. A waveguide with a generally circular or elliptical cross section provides the antenna with dual polarization capability.
Electrical performance of a dual polarized reflector antenna with a self supported feed is typically measured with respect to gain, cross polarization, edge illumination and return loss characteristics.
Prior reflector antenna feed assemblies typically comprise a sub reflector attached to a waveguide by a dielectric block that positions the sub reflector at a desired orientation and distance from the end of the waveguide. Alternatively, the reflector antenna may focus the signal upon a feed horn formed at a waveguide end or a separately supported sub reflector that then focuses the signal upon a feed horn/waveguide. When a separate feed horn configuration is used, a dielectric cover, radome or other environmental seal is applied to protect the open end of the waveguide.
The interfaces between the environmental seal(s), dielectric block, waveguide, sub reflector and any adhesives or mechanical interlocks used to secure the components together create impedance discontinuities that are significant sources of return loss. Also, the metal waveguides are typically structural elements with a significant thickness, creating edge radiation characteristics that contribute to the generation of backlobes in the antenna signal pattern.
U.S. Pat. No. 6,919,855 issued Jul. 19, 2005 to Hills, assigned to Andrew Corporation as is the present invention, describes dielectric blocks incorporating corrugations in the dielectric surface for pattern and return loss optimization. A subreflector is formed by metalizing the desired subreflector surface of the dielectric block.
U.S. Pat. No. 6,985,120 issued Jan. 10, 2006 to Lewry et al., assigned to Andrew Corporation as is the present invention, describes a reflector antenna with a self supported feed assembly formed as a hollow dielectric waveguide and cone coupled at the narrow end to the reflector dish and at the wide end joined to a sub reflector surface. Formed via injection molding from a dielectric material, the waveguide and sub reflector surfaces have a thin metallic surface coating to contain and reflect radio frequency signals. However, a slight taper along at least the waveguide inner diameter, to improve injection molding mold separation, degrades the electrical performance. Also, the thickness of the dielectric along the cone and waveguide portions is a trade off between strength and an impedance discontinuity that is difficult to match for, without adding an additional impedance matching element.
Competition within the reflector antenna industry has focused attention on antenna designs that reduce antenna materials and manufacturing costs but which still satisfy and or improve upon stringent electrical specifications,
Therefore, it is an object of the invention to provide an apparatus that overcomes deficiencies in the prior art.